Nanoscience, sometimes called “nanotechnology” or “nanotech,” refers to a field focused on the control of matter on an atomic and molecular scale. Generally, nanoscience deals with structures 100 nanometers or smaller, and involves developing materials or devices within that size. The potential applications for nanotechnology are extremely diverse and include, for example, extensions of conventional device physics, new approaches based upon molecular self-assembly, development of new materials with dimensions on the nanoscale, prolongation of cell viability and biological labeling to study intracellular trafficking and organelle functions and other everyday applications such as photography, reaction catalysis, optoelectronics and information storage. The potential to create many new materials and devices with wide-ranging applications, such as in medicine, electronics, and energy production, renders nanoscience a popular field of study. However, nanotechnology raises many of the same issues that come with the introduction of any new technology including concerns about the toxicity and environmental impact of nanomaterials.
Ionic silver (such as silver nitrate) has long been known as an antimicrobial or antifungal agent with the power to kill bacteria and other germs. In fact, ancient Greeks and Romans frequently used silver as an antiseptic in the pre-antibiotic era and even kept their liquids free from contamination by placing the liquids in silver jars. However, ionic silver was abandoned for such uses due to its cytotoxicity and adverse effect on human and animal health and has been limited to non-health related uses such as photography. Recently, however, a number of studies have reported the efficacy of silver nanoparticles as an antimicrobial against bacteria such as E. coli and even against viruses such as the Human Immunodeficiency Virus and it has been shown that silver particles are 100 times more effective than silver salts as antiseptics. It would therefore be desirable to prepare silver nanoparticles or silver-alloy nanoparticles to maximize the antimicrobial properties of silver. Moreover, recent studies have shown that, in proper concentrations, silver nanoparticles are not dangerous to humans when used externally.
Various methods have been employed for the synthesis of nanoparticles of metallic origins including co-precipitation methods in aqueous solutions, electrochemical methods, aerosol, reverse microemulsion, chemical liquid deposition, photochemical reduction, chemical reduction in solution, and UV radiation. For example, conventional methods have included the reduction of Ag+ with sodium borohydride, aldehyde, hydrazine, or phenylhydrazine with alkylamine as reductants and have also included synthesis via a solvothermal process. However, solvothermal synthesis typically relies on multiple reagents and requires controlled conditions which can be tedious and troublesome.
Moreover, all of these methods have limitations in controlling the particle size and production of particles on an industrial scale. Capping reagents such as poly (N-vinyl-2-pyrrolidone) (PVP) are commonly used to regulate the size and morphology of the nanoparticles synthesized. However, the strong hydrophobicity of many capping reagents requires an organic solvent such as acetone, methanol, or toluene in order to counteract the hydrophobicity. Chelating agents have also been used for controlling the aggregation of nanoparticles, but many of these agents and the reagents described hereinabove are toxic and therefore harmful to humans and the environment. In order to eliminate the use of such hazardous organic chemicals, a hydrothermal process has been developed which involves Ag NP synthesis by reduction of Ag+ using β-D-glucose inside the nanoscopic template formed by a starch. However, this method requires at least twenty hours of incubation and uses a starch as stabilizing reagent. A costly microwave-digestion system-assisted synthesis using polyol has also been reported as accelerating the liquid-phase reaction in synthesizing nanoparticles.
Thus, most of the currently available methods require the use of hazardous organic solvents, stabilizers, capping reagents, the purging of reaction vessels with inert gases, and/or prolonged incubation, even with the use of expensive microwave synthesizers. In addition, the presence of stabilizers and capping reagents interferes with the purification of nanoparticles and reduces the yield thereby raising the risk of presence of silver ion (Ag+) as a contaminant. Moreover, the use of organic reagents, organic solvents, organic-aqueous mixtures, organic capping reagents, and the disposal of unreacted silver ion result in increased, costly, and hazardous waste. It is therefore desirable to provide a method of preparing silver nanoparticles that does not include the use or the production of hazardous materials and that is less costly to produce.